This invention relates to the operational control of a digital computer system, and more particularly, to the digital logic circuitry for indicating the end of an operand data string stored in a temporary storage memory, wherein the operand data string can be multiple variable length operands.
An objective, which almost always faces designers furthering the advancement of digital computers, is to decrease the time required for executing each of the instructions executed by the digital computer, thereby decreasing the overall time required by the digital computer to perform a predefined task and increasing the efficiency of the digital computer. Many schemes have been devised by digital computer designers in an attempt to meet this objective. In the execution of some instructions, the operand data stored in temporary storage memory must be read. The temporary storage memory has associated therewith, a read address register which contains a read address value specifying a location of the temporary storage memory to be read. Although the reading of the operand data requires some number of cycles, the storing of the resulting data may require more cycles than were required for the reading of the operand data. An example of such an operation is when two words are to be read and a move, or shift, operation of less than a full word (or double word) is performed. The results of the operation require three write operations into a results stack, since part of the second word is shifted to a third word. During the additional cycle (or cycles) it is desirable to freeze the read address value contained in the read address register to prevent reading data associated with a subsequent instruction. The data available indicator of the present invention generates a signal which freezes the contents of the read address register when the end of the operand data string is detected, i.e., the signal inhibits the read address value from any further changes and permits additional cycles to be generated for completing the execution of the instruction.